Nuclear magnetic resonance gyroscope

ABSTRACT

A nuclear magnetic resonance gyroscope which derives angular rotation  thef from the phases of precessing nuclear moments utilizes a single-resonance cell situated in the center of a uniform DC magnetic field. The field is generated by current flow through a circular array of coils between parallel plates. It also utilizes a pump and read-out beam and associated electronics for signal processing and control. Encapsulated in the cell for sensing rotation are odd isotopes of Mercury Hg 199  and Hg 201 . Unpolarized intensity modulated light from a pump lamp is directed by lenses to a linear polarizer, quarter wave plate combination producing circularly polarized light. The circularly polarized light is reflected by a mirror to the cell transverse to the field for optical pumping of the isotopes. Unpolarized light from a readout lamp is directed by lenses to another linear polarizer. The linearly polarized light is reflected by another mirror to the cell transverse to the field and orthogonal to the pump lamp light. The linear light after transversing the cell strikes an analyzer where it is converted to an intensity-modulated light. The modulated light is detected by a photodiode processed and utilized as feedback to control the field and pump lamp excitation and readout of angular displacement.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

This invention relates to a nuclear magnetic resonance gyroscope and inparticular to a nuclear magnetic resonance gyroscope which utilizesparallel plate magnetic field coils to provide a uniform DC magneticfield about a resonance cell and transverse pumping of the resonancecell.

Current nuclear magnetic resonance gyroscopes incorporate resonancecells, pump and readout lamps, optics, and associated electronics forcontrol and signal processing. Isotopes are encapsulated in theresonance cell centrally positioned in a DC magnetic field generated bya hemmholtz or cylindrical field coil and a current source. Theprecession of the nuclear magnetic moment is sustained by an AC magneticfeedback field. The pump lamp is comprised of a single isotope which isexcited to produce the light required for the optical pumping of theresonance cell. The readout lamp is identical in construction to thepump lamp and is used for determing angular changes. The techniques usedin the readout process are the Faraday and Dehmelt. For both techniquesintensity modulated light is detected by a photodetector. The signal isamplified, conditioned, and demodulated to produce the correct signalfor control and information processing. Degradation of performance ofexisting nuclear magnetic resonance gyros occurs because limitedmagnetic field uniformity is provided in both the transverse andlongitudinal direction; external magnetic fields couple with the DCmagnetic field of the gyro altering the direction of the sensitive axisfrom that defined on the gyro case; phase shifts are introduced by achange in angle between the feedback field, light beam direction and DCmagnetic field; and the feedback field interacts with the atomicsublevels of the isotopes reducing their relaxation time and affectingperformance.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a nuclearmagnetic resonance gyro having a uniform magnetic field. It is anotherobject of the invention to provide improved shielding so that externalmagnetic fields do not couple with the DC magnetic field and alter thedirection of the sensitive axis. It is a further object of the inventionto eliminate changes in angle between the feedback field, the directionof the light beam and the DC magnetic field. It is still a furtherobject of the invention to provide a nuclear magnetic resonance withgyro with no feedback field, thereby preventing a reduction in therelaxation times of the isotopes.

Briefly, these and other objects of the invention are achieved asfollows:

A resonance cell, utilizing Mercury isotopes Hg¹⁹⁹ and Hg²⁰¹encapsulated therein for sensing rotation, is centered in a parallelplate magnetic field coil. The coil structure produces an extremelyuniform field in the region of the sample cell. It also serves as anadditional shunt path for external fields. A pump lamp is excited by anRF oscillator and maintained at threshold by the use of a poweramplifier. A feedback loop controls the gain of the power amplifierthereby modulating the light intensity at the precessional frequency ofthe nuclei (Lamour frequencies) for optical pumping. The modulatedunpolarized light beam is transformed into modulated circular polarizedlight by directing it through a linear polarizer, quarter-wave platecombination. The circularly polarized light is then redirected by amirror to the resonance cell for optical pumping along an axistransverse to the magnetic field. A readout lamp is excited in a similarmanner as the pump lamp. The unpolarized light beam is directed througha set of lenses and a linear polarizer to produce linearly polarizedlight. The linear polarized light is redirected by a mirror to theresonance cell along another axis transverse to the magnetic field andorthogonal to the pump beam. The readout beam's plane of polarization iscontinuously varied by the motion the nuclei in the resonance cell. Thisoscillation is transformed into intensity-modulated light by a suitablyoriented analyzer. The intensity-modulated light is then detected by aphotodetector and utilized in the control and signal processingelectronics for field and pump lamp control and angle processing.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a nuclear magnetic resonance gyroscope according to theinvention;

FIG. 2 is a top view of the nuclear magnetic resonance gyroscope shownin FIG. 1;

FIG. 3 is a cross-sectional view of the nuclear magnetic resonancegyroscope taken along the line 3--3 of FIG. 2;

FIG. 4 is a cross-sectional view of the nuclear magnetic resonancegyroscope along the line 4--4 shown in FIG. 2;

FIG. 5 is a cross-sectional view taken along the line 5--5 of FIG. 3;

FIG. 6 is a cross-sectional view taken along the line 6--6 of FIG. 3;

FIG. 7 is an exploded top view of the parallel magnetic plate field coilaccording to the invention;

FIG. 8 is a partial cross-sectional view of the parallel magnetic platefield coil taken along the line 8--8 of FIG. 7;

FIG. 9 is a schematic diagram of pump and readout lamp light beam pathsand a block diagram of the electronic control circuitry according to theinvention; and

FIG. 10a, is a diagrammatical model of a nucleus indicating thedirection of an angular momentum vector resulting from application of aDC magnetic field according to the invention;

FIG. 10b is the nucleus as shown in FIG. 10a indicating the direction ofthe angular momentum vector resulting from application of a pump lamplight beam and the D.C. magnetic field; and

FIG. 10c is the nucleus of FIG. 10b indicating the direction of theangular momentum vector resulting from application of a readout lamplight beam, the pump lamp light beam; and the D.C. magnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, wherein like reference charactersdesignate like or corresponding parts throughout the sevral views, thereis shown in FIG. 1 a nuclear magnetic resonance gyro having threemodular sections; a resonance cell container 10, a lens stack container40 and a base container 60.

Cell container 10 shown in FIGS. 3, 4 and 5 has mirror mounts 14a and binserted through the walls thereof positioned orthogonal to each otherbetween cover 12 and lens stack container 40. Mount 14a is centered on areadout beam R, axis X, passing through the center of a resonance cell34 and mount 14b is centered on a pump beam P axis Y orthogonal to the Xaxis and also passing through the center of resonance cell 34. A Z axisis defined as passing through the center of cell 34 and perpendicular toboth the X and Y axis. Positioned in the center of cell container 10 isa cell cage 24 having inserted in the center thereof on the X axis acylindrical spool 26. Spool 26 is used to secure the position of aresonance cell 34 in the center thereof. Rotation and alignment of thesymmetric axis of cell 34 is achieved by positioning spool 26 afterwithdrawing brake rod 37. Resonance cell 34 comprises a transparentsealed quartz-pyrex envelope containing mercury vapor of odd isotopesHg¹⁹⁹ and Hg²⁰¹. Cell 34 is centrally positioned in a DC magnetic fieldH_(o) generated by an electronically computer regulated stable currentsource which maintains field H_(o) constant. Spool 26 having a hole 27longitudinally therethrough for allowing the readout beam R to passthrough to cell 34 and having at one end therein a threaded portion 27afor insertion of an alignment tool (not shown) in place of photodetector38. A U shaped notch 28 is located in the center of spool 26 in linewith the Y axis for permitting pump beam P to strike cell 34. A cavity29 is located below U shape notch 28 and the center of spool 26 forholding cell 34 envelope sealing point. Spool cage 24 has a holetherethrough aligned on the X axis in which spool 26 is positioned.Brake rod 37 is positioned and held against spool 26 for maintaining theposition of cell 34. Cage 24 with spool 26 and cell 34 therein isinterposed between parallel plates 16a and b. Plates 16a and b arefabricated from a material having a high permeability such as siliconcore c iron having a permeability equal to 6,000. Proper separationbetween the upper and lower plates 16a and b respectively are maintainedby cage 24. Cage 24 is cylindrical and has a lip extending above andaround the perimeter of the top and bottom sides. FIGS. 7 and 8 areexpanded views of the parallel plate magnetic field coils. FIG. 8 inparticular shows a cross-sectional view illustrating the magnetic fluxpath φ through the parallel plates 16b across the air gap to parallelplate 16a through 16a to the top of coil 22. Plates 16a and 16b are diskshaped but have an area 19a and 19b about the center which is thickerthan the outer periphery area of the disk shaped plates. Said thickerportions of plates 16a and 16b fit snuggly within the circumferentiallip on the top and bottom of cage 24 respectively. The increasedthickness of the center area of plate 16a and b provide an addition fluxpath to concentrate the magnetic field in the central area than would aflat disk. Parallel magnetic plates 16a and 16b are directly connectedtogether by eight coils 22 each coiled about an adjustable core 18 andsymmetrically located about the perimeter of plates 16a and b. Coils 22are connected in a series string and the current therethroughautomatically regulates the DC magnetic field. Coil 22 wire is capableof withstanding 300 degrees C. Core 18 is comprised of high permeabilitymaterial such as silicon core C iron and is internally threadedtherethrough having an adjustable magnetic center rod 20 which can beturned into or out of core 18 to change the permeability thereof andhence the magnetic field H_(o). The adjustment provides the ability tofine tune the field H_(o) in the central area of plate 16a and b whichhas magnetic irregularities due to nonuniformity in material. Grooveshave been cut into the well of container 10 where the parallel plate 16brests for a heater coil 17. Heater coils 17 are also placed over plate16a to provide symmetrical heating of the resonant cell 34. Heater coil17 provides 600 watts at turn on. Coil structure 18 is positioned withinthe cell container 10 so that the axis of symmetry of hole 27 is alignedalong axis X. Mounts 14a and 14b inserted through the walls of container10, orthogonal to each other, extend partially into shafts 15a and 15brespectively. The center of 14a and 14b lie on the axis X and Yrespectively. Mirrors 25a and 25b are attached to mountings 14a and 14brespectively and are positioned to reflect light received from lens well42a and 42b onto the X axis and Y axis respectively. Analyzer 36 ispositioned at 57.40 degrees away from the perpendicular with respect tothe direction of propagation of readout beam R provides the intensitymodulated light I for detection by a photodetector 38 positioned on theX axis opposite of mirror 25a. Cell container 10, cage 24, spool 26,mirror mounts 14a and b and cover 12 are made of a ceramic materialwhich requires no firing after machining and has physicalcharacteristics conclusive to a gyro such as an extremely low magneticsusceptibility material. Mirror mounts 14a and b and respective mirrors25a and b are used for directing the readout beam R and a pump beam Palong the transverse axes into cell 34.

A lens container 40 as shown in FIGS. 1, 3 and 4 has two threadedcylindrical wells 42a and 42b for containing in a stack configurationall the components necessary to produce circularly polarized light foroptical pumping and linearly polarized light for readout respectively.The optical components threadingly stacked in well 42b are a quarterwave plate 44b, a polarizer 46b, a 2537 Angstrom filter 48b, abi-concave lens 50b, an upper condenser 52b, and a lower condenser 54b.Well 42a contains threadingly stacked therein a polarizer 46a abi-concave lens 50a, an upper condenser lens 52a, and a lower condenserlens 54a. Lens container 40 is fabricated from a non-magnetic reactingmaterial such as a vulcanized fiber.

Base block 60 comprises lamp housing 62a and 62b each partially insertedthrough the wall of container 60 orthogonal to each other. Housing 62ais centered on the X axis and 62b on the Y axis. A pump lamp 66acomprises a quartz envelope having the shape of a miniature dumb bellhaving two spherical sections connected therebetween by a capillarysection encapsulating an isotope whose spectral characteristic matchthose of the isotopes in cell 34. Pump lamp 66a is maintained inposition at one spherical end by a hole 64a and at the other sphericalend by an excitation coil 68a coil thereabout. Excitation coil 68areceives current from a gain varied power amplifier 100. A readout lamp66b having an encapsulated isotope has the same configuration as lamp66a is positioned in housing 62b at one spherical end in hole 64b and atthe other spherical end by an excitation coil 68b coiled thereabout.Light intensity of beams P and R in wells 42a and 42b are adjusted bydiaphragms 69a and 69b respectively. The unpolarized intensity modulatedpump light beam P from pump lamp 66b is directed through combination oflenses 54b, 52b, 50b, filter 48b, linear polarizer 46b and quarter waveplate 44b producing circularly polarized light and reflected from amirror 25b onto axis Y into cell 34 transverse to field H_(o) foroptically pumping. Unpolarized readout light beam R from readout lamp55a is directed through the lenses in well 42a through lenses 54a, 52a,and 50a and linear polarizer 46a and reflected from mirror 25a onto axisY through cell 34 transverse to field H_(o) and orthogonal to pump beamP along axis Y. Lamps 66a and 66b have their center capillary portion,aligned with the optical axis of the lens stacks in well 42a and 42brespectively. Diaphragm 69a and 69b positioned above lamps 66a and 66bare used to restrict the amount of light into lens wells 42a and b. Basecontainer 60 and the lamp housings 62a and 62b are constructed from anon-magnetic fibrous material such as an acetyl resin. The isotopeencapsulated within the readout lamp 66b has spectral characteristicsdifferent from the spectral characteristics of resonant cell 34 toensure minimal absorption of the readout light beam R. The readout lamp66b is excited from a 100 mHz clock 102. As shown in FIG. 10c readoutbeam R interacts with the precessing nucleus of a mercury atom rotatingits plane of polarization. An analyzer 36 is positioned at a 57.40degrees angle away from the perpendicular with respect to thepropagation of beam R to convert the translated linearly polarized lightfrom cell 34 into intensity modulated light. A photodetector 38 isconnected through the wall of container 10 on the X axis opposite mirror25a for receiving intensity modulated light I and detecting themodulating signal Am and providing it as voltage fluctuations toelectronic circuit 70.

Electronic circuit 70 separates two precessional frequencies, uses themto control pump lamp excitation and gating of a high frequencyoscillator 84 into counting chains 78 and 90. The resultant countsrepresent the Lamour frequencies plus vehicle rotation rate to aresolution of microhertz. A computer 82 is connected to receive theresultant counts through a multiplexer 80 for processing and providingfeedback to control the D.C. magnetic field H_(o). Electronic circuit 70shown in FIG. 9 is connected as follows. A phase stable amplifier 72comprising a voltage amplifier having no phase shift between input andoutput is connected to receive the detected modulation signal Am fromphotodetector 38 and to provide a composite signal C having two mainfrequency components consisting of 369 Hz and 1000 Hz and deviationstherefrom resulting from the precessing Mercury isotopes in cell 34. Alamour filer circuit 74 is connected to receive and separate compositesignal C into the two main components. A sample rate multiplier 76 isconnected to receive both components, convert them from a sinewave to asquare wave of the same frequencies, provide two disable signals D₁,each having a period one hundred times longer than the input signals,and provide an enabling signal E_(mi) to a computer 82. An angle counter78 is connected to receive signal D₁ and is used to count a 200megahertz clock pulse from an oscillator 84 for the time period derivedby multiplier 76. Angle counter 78 provides a count signal N₁ equivalentto the number of pulses it received before receiving disable pulse D₁when enabled from computer 82 by signal E_(cl). A count multiplexer 80is connected to receive count signal N₁ and to transfer it to thecomputer 82. Computer 82 can be a general purpose computer programmed toreceive count signal N, and perform arithmetic operations to calculateand provide the angle of rotational motion about a sensitive axis asdefined by the direction of H_(o) (Z axis).

A frequency multiplier 86 is connected to receive the two frequenciesand then by a multiplication technique well known in the art produce thesum frequency signal S all others being filtered. Signal S used tocontrol H_(o) field. An H_(o) sample controller 88 is connected toreceive signal S, provide a disable signal D₂ that is 100 times theperiod of the incoming signal S and an enable signal E_(m2) to computer82. The period multiplication increases the resolution of the finalchange in coil 22 current. An H_(o) control counter 90 is connected toreceive 200 mHz oscillator signal from oscillator 84 and disable signalD₂. When signal D₂ occurs counter 90 stops incrementing and provides thecount signal N₂ when enabled from computer 82 by signal E_(c2). Computer82 is connected to receive count signal N₂ and provide a feedback signalF_(d) consisting of a 16 bit word indicative of the parallel platemagnetic field coil current control. A digital-to-analog converter 92 isconnected to receive signal F_(d) to provide a feedback current F_(a)which will increase/decrease the current I_(p) from the power amplifier94 to the coil 22 so as to maintain the sum frequency signal S constant.A pulse forming circuit 96 such as a Schmidt trigger is connected toreceive the precessional frequencies components from Lamour filtercircuit 74 and to provide a trigger pulse T, at the rate of thecomponent frequencies. A phase control circuit 98 is connected toreceive pulse T for providing a gating pulse G_(p) having apredetermined pulse width and duty cycle to excite pump lamp 66b at theproper time and for the proper duration to be in phase with theprecessing moments of cell 34 nuclei. Lower amplifier 100 is connectedto receive a 100 mHz excitation signal O and to provide a thresholdlevel excitation and periodically pulsed by gating pulse G_(p) increaseexcitation power. Pump lamp excitation coil 68b is connected to receivethe gain varied excitation signal O from pump amplifier 100. Readoutlamp excitation coil 68a is connected to receive excitation signal Ocontinuously from 100 mHz oscillator 102.

A triple nested magnetic shield not shown surrounds the gyroscope. Theshield cylindrical axis of symmetry is aligned along the symmetric axis(Z axis) of the gyroscope. The shield provides attenuation of allexternal fields to a nominal value which shall not affect theperformance of the instrument.

Operation of the nuclear magnetic resonance gyroscope utilizes theintrinsic property of certain nuclei to determine angular displacementabout a defined input axis. The nuclei have angular momentum, hence, amagnetic moment. When the nuclei are placed in a weak magnetic field,they precess about the direction of the field as defined by equation 1.

    ω=γH.sub.o                                     Equation 1

ω_(L) is the precessional frequency;

γ is the gyromagnetic ratio, and H_(o) is the DC magnetic field whosedirection defines the input axis of the gyroscope. As illustrated inFIG. 10, field H_(o) lies on the Z axis. An ensemble of aligned nucleimust be established in order to obtain a detectable signal. This isperformed by a technique known as optical pumping. A beam of circularlypolarized light is produced and directed at the resonant cell containingthe atoms in a gaseous state. The light of proper wavelength is absorbedby the atoms. The atoms then align themselves along the direction of thepumping beam. According to the invention, the pump beam is directedperpendicular to the direction of the magnetic field H_(o) asillustrated in FIGS. 9 and 10. The reoriented atoms now in a positionperpendicular to H_(o) experience a torque which causes them to precessabout field H_(o). This is a free precession gyroscope. A feedbacksignal is established through the readout circuit and is utilized tomodulate the pump lamp intensity. In this manner, the pump lamp is inphase with the precessing ensemble replacing those nuclei which havelost their coherence due to decay processes going on in the resonancecell. This establishes an equilibrium ensemble or net magnetc momentcapable of producing a detectable signal. Readout of the net magneticmoment is achieved by utilizing the Faraday technique. A beam oflinearly polarized light in the same plane as the pump beam beingorthogonal is directed at the resonant cell 34. As the linear lighttraverses the cell, the plane of polarization varies due to its positionwith respect to the precessing net magnetic moment. The light is thentransformed into intensely modulated light by an analyzer 36 and issensed by a photodetector 38. The observed signal of ω₁ is given byequation 2.

    ω.sub.1 =ω.sub.L +ω.sub.R                equation 2

where ω_(R) is vehicle rotation sensed by the gyroscope along thedirection of field H_(o). If a single nuclei were to be used, it wouldbe extremely difficult to produce accurate angular measurements becauseof the stability requirement that would have to be imposed on H_(o)field. To overcome this difficulty, two nuclei are placed in a singlecell. The detected signal is then the sum of the individual signal fromthe cell given by equation 3.

    ω=ω.sub.1 +ω.sub.2                       equation 3

where ω₂ is the precessional frequency of the second nuclei and isidentical to equation 2 except for a change in sign for ω_(R). Utilizingequation 3 and substituting in 1 and 2, the field control is expressedby equation 4.

    H.sub.o =(ω.sub.1 +ω.sub.2)/(γ.sub.1 +γ.sub.2) equation 4

Field control is a matter of measuring ω₁ +ω₂ and processing thisinformation with the predetermined value of α₁ and α₂. Electronically,the gyro output signal is separated then beat together to produce thesum and difference. The difference frequency is filtered leaving the sumwhich is independent of ω_(R). The sum signal is then utilized tocontrol an accounting chain from an extremely stable high frequencycrystal oscillator. After the counting cycle is completed, theinformation is multiplexed into a computer for use in controlling adigital to analog converter for controlling the current field H_(o).This eliminates the requirement for an ultra stable current sourcerequired in a single nuclei approach. Vehicle angular information isbased upon a difference between ω₁ +ω₂ and is given by equation 5.##EQU1## Equation 5 was based upon equation 2 and identical quation forω₂. The difference frequency is utilized in the same manner as the sumfrequency in the field H_(o) and sum frequency in the H_(o) controlcircuitry. The information after the gating is completed is multiplexedby multiplexer 82 into computer 82 and a calculation is performed todetermine ω_(R).

Pump beam lamp 66b is excited by current supplied from pump amplifier100 through winding 68b. Upon excitation, unpolarized light is directedthrough diaphragm 69b through the lens polarizer and quarter wave platesstacked in lens well 46b. Now circularly polarized light beam P isreflected from mirror 25b onto the Y axis into resonant cell 34.

Readout lamp 66a obtains excitation from 100 mHz clock 102 and emitsunpolarized light through the lens stack in lens well 46a. Linearlypolarized light beam R exits well 46a and is reflected from mirror 25aonto the X axis through the resonant cell 34 to analyzer 36 and then asamplitude modulated light to photodetector 38. The voltage fluctuationfrom detector 38 are separated into the two precessional frequencies andutilized to control pump lamp 66b excitation, the current flow throughcoils 22 and provide vehicle rotation rate.

Some of the many advantages of the present invention should now bereadily apparent. The invention provides parallel plate magnetic fieldcoil which shunts external magnetic fields and produces an extremelyuniform field in the region of the resonant cell. The invention uses atransverse AC pumping technique, thereby eliminating the effect of phaseshifts which occur between the AC feedback magnetic field and theprecessing nuclei, eliminating the angle change between the feedbackmagnetic field, light beam direction and the DC magnetic field, andreducing the relaxation time due to the interaction of the AC feedbackmagnetic field with the atomic sublevels.

Obviously, many modifications and variations of the present inventionare possible in view of the above teaching. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A nuclear magnetic resonance gyroscope whichderives rotational information from the phases of precessing nuclearmoments for determining the angle of rotation of the gyroscope about apredetermined axis comprising:a resonance cell containing samples ofisotopes of an element for producing nuclear magnetic resonance signalsis mounted on the predetermined axis; plate means including a pair ofmetallic plates for producing a uniform unidirectional magnetic fieldoriented along the predetermined axis and intersecting said resonancecell for causing free precession of the net magnetic moment therein,said plates being spaced therebetween by a plurality of radially spacedcoils symmetrically positioned around the perimeter of said plates, eachof said coils being wound about a permeability adjustable metallic corewith the lateral axis aligned with the predetermined axis intersectingthe resonance cell located within said plate means; first optical meansfor periodically increasing the magnitude of the net magnetic moment ofsaid resonance cell transverse to said magnetic field; second opticalmeans transverse to said magnetic field and orthogonal to said firstoptical means for detecting the phase of the magnetic moment of saidresonance cell; output means for processing said detected phases toproduce an output signal indicative of the rotation of said gyroscopeabout said predetermined axis, a feedback signal for controlling saidfirst optical means, and a second feedback signal for controlling saidmagnetic field.
 2. The gyroscope as set forth in claim 1 wherein saidplate means further comprises a pair of metallic disks having an areaabout the center thereof having increased thickness for providing anadditional flux path for said magnetic field to intersect said resonancecell.
 3. Apparatus for generating a uniform magnetic field comprising,in combination:a pair of spaced coaxial circular plates of magneticmaterial, each plate having a raised circular and planar surfaceparallel to and confronting the raised surface of the other plateforming a uniformly thicker portion about the center; a plurality ofparallel magnetic coils connected at their ends between said plates andsymmetrically spaced about the thinner portion of said plates forgenerating a magnetic field in the space between said surfaces; and aplurality of magnetic rods inserted in one end of respective ones ofsaid coils, the insertion length being adjustable for varying the fluxdensity in the space between said surfaces to obtain a uniform magneticfield.
 4. Apparatus according to claim 3 wherein each of said rods arethreadingly inserted in said coils for altering the permeabilitythereof.